The present invention relates to an industrial vehicle having a tiltable mast which supports a loading attachment and guides a movement of the attachment, more particularly to a device for measuring a load weight moment in back-and-forth direction of such an industrial vehicle.
A forklift truck as an industrial vehicle has a pair of masts each including outer and inner masts so that the masts can extend upward. The masts, which are mounted on the front portion of the truck body, support a fork by means of a lift bracket slidably provided between the masts. A lift cylinder provided on the truck raises and lowers the lift bracket together with the fork along the masts, up to the top of the fully extended masts. The forklift truck further includes tilt cylinders. The tilt cylinders tilt the masts forward and backward with respect to vertical positions of the masts. The tilting action of the masts makes the loading work easy and stabilizes the forklift truck.
However, when the fork is loaded, a gravity center of the forklift truck moves forward. And a moment of the load acting on the masts becomes large when the fork is raised higher by the extended masts. If the mast with the fork loaded is tilted forth, the center of gravity moves forth further, causing the stability in the longitudinal direction (back-and-forth direction) of the forklift truck to be worsened. On the other hand, if the mast with a loaded fork is tilted backward together with the center of gravity, front wheels of the truck may tend to be raised and to possibly slip. Therefore, in a conventional forklift truck, a tiltable angle range of the mast in both back and forth directions of the truck are fixed at certain values.
In case that the load is placed at a higher location, the mast has to be tilted forth while the fork is raised higher. At this time, if the mast is mistakenly tilted forth at high speed, the load may be crumbled or rear wheels of the truck may float. That is, the forklift truck is in unstable condition, especially, in its longitudinal direction. Therefore, operators of the truck has to carefully incline the masts at low speed by inching operation to avoid too much forward inclination of the mast, whereby the operators are stressed mentally very much.
To resolve the above problems, there is a forklift truck which stops forward tilting motion of the masts, or whose alarm means goes off when a load eight moment detected through the tilt cylinder approaches unstable condition of the forklift truck. In a conventional art as shown in FIG. 7, a method for measuring moment in a longitudinal direction of the forklift truck is known, as follows.
A pressure sensor 54 is provided to sense pressure of hydraulic fluid in a rod side chamber of a tilt cylinder 53 which tilts a mast 52 of the forklift truck 51. Based on the detected pressure by the sensor 54, load weight moment M is calculated by the following equation.
M=2FL
In the equation, the numeral xe2x80x9c2xe2x80x9d means to double thrust or axial force of the tilt cylinder 53 because the forklift truck has two tilt cylinders mounted on both left and right sides of the truck. The letter xe2x80x9cFxe2x80x9d represents the axial force of the tilt cylinder calculated by multiplying the tilt pressure and pressured area of the tilt cylinder 53. The letter xe2x80x9cLxe2x80x9d represents the distance between a rotational center of front wheels 58 and the longitudinal axis of the tilt cylinder 53.
The pressure sensor 54 is disposed on a conduit 57 connecting a control valve 56, which controls supply of the hydraulic fluid to the tilt cylinder 53 based on operation of a tilt lever 55, to the rod side chamber of the tilt cylinder 53. The pressure sensor 54 is arranged on either one of the conduits 57 each connected to their respective tilt cylinders 53 because an equal pressure acts on each of the tilt cylinders 53 mounted on both the left and right sides of the forklift truck.
However, it is difficult for the conventional forklift truck to continuously detect the accurate pressure corresponding to the load weight W because the pressure detected by the sensor 54 dose not always accurately reflect the load weight W on the fork 59 of the forklift truck 51.
For example, when the control valve 56 is switched to its neutral position from its forward tilting position by manipulation of the control valve 56 at the time the mast 52 is tilting forth, extra pressure corresponding to acceleration of the tilting mast 52 may be involved within the conduit 57. As a result, the pressure sensor 54 detects the pressure more than exact pressure corresponding to the load weight W. On the other hand, when the control valve 56 is switched from the neutral position to the forward tilting position, the hydraulic fluid acts on the bottom side room of the tilt cylinder. As a result, the pressure sensor 54 detects the pressure more than exact pressure corresponding to the load weight W because the pressure acting on the bottom room is added to the pressure corresponding to the load weight W.
Moreover, when the mast 52 reaches its maximum forward tilting position which means a stroke end of the tilt cylinder, no pressure acts on the rod side chamber of the cylinder. As a result, the pressure sensor 54 does not detect any pressure corresponding to the load weight W. When the mast 52 reaches its maximum backward tilting position, another stroke end of the tilt cylinder, the maximum pressure set by a relief valve acts on the rod side chamber. As a result, extra pressure larger than exact pressure corresponding to the load weight W is detected by the pressure sensor 54.
It is an object of the present invention to provide a device for measuring a load weight moment in back-and-forth direction of an industrial vehicle without affection of a control valve manipulation controlling fluid flow to a tilt cylinder which tilts a mast of the vehicle.
It is another object of the present invention to provide a device for continuously measuring a moment in back-and-forth direction of an industrial vehicle without detecting pressure in a tilt cylinder when a control valve controlling fluid flow to the tilt cylinder is switched from its neutral position to its forward or backward tilting position such that the tilt cylinder reaches its forward or backward stroke end.
To attain the above first object, an industrial vehicle according to the present invention comprises first and second pressure sensors which detect pressures in both a rod side chamber and a bottom side chamber of a tilt cylinder. Detected signals from the both sensors are used for calculating thrust or axial force of the tilt cylinder. A load weight moment in back-and-forth direction of the vehicle is calculated based on the thrust force calculated from the pressures in both the rod and bottom side chambers.
According to the present invention, the thrust of the tilt cylinder is calculated by the following equation.
F=P1S1xe2x88x92P2S2
Letters xe2x80x9cP1xe2x80x9d and xe2x80x9cP2xe2x80x9d each denote pressures in the rod side chamber and the bottom side chamber of the tilt cylinder, respectively. Letters xe2x80x9cS1xe2x80x9d and xe2x80x9cS2xe2x80x9d denote areas receiving the pressures in the rod side chamber and the bottom side chamber of the cylinder, respectively. A letter xe2x80x9cFxe2x80x9d denotes the thrust force. According to the formula, it is clear that affection of the pressure P2 in the bottom side chamber exerting upon the pressure P1 in the rod side chamber is offset or cancelled. As a result, the thrust force F corresponding to the load weight W is accurately calculated. The load weight moment in the back-and-forth direction of the vehicle is then calculated by multiplying the thrust force F and the distance L between a center of a front wheel and a longitudinal axis of the tilt cylinder.
Preferably, the first pressure sensor is arranged in the first conduit connected to the rod side chamber, and the second pressure sensor is arranged in the second conduit connected to the bottom side chamber of the cylinder. The calculation may be corrected by correcting means which compensates pressure losses within the first and second conduit. Therefore, the correcting means compensates the pressure losses in the first conduit and in the second conduit, then, the pressure in the rod side chamber and in the bottom side chamber of the cylinder are detected accurately, even though the pressure loss of the hydraulic fluid flowing in the first or second conduit becomes an error.
Correction values used for the correcting means may be represented by a function of the tilt cylinder in its operating condition. The correction value can be changed by the function of the tilt cylinder in active condition according to the tilting speed of the mast and the direction of the tilting motion, the pressure loss, which occurs in the first conduit to the rod side chamber of the cylinder or in the second conduit to the bottom side chamber of the cylinder, is easily corrected, even though the direction and the speed of the hydraulic oil flowing in the first or second conduit changes.
The present invention is further equipped with a stroke end sensor which detects the stroke end of the tilt cylinder, a weight sensor which detects the load weight on the loading attachment and a height sensor which detects the lifting height of the loading attachment. At the stroke end position of the cylinder, it is determined whether the lifting height is within a certain predetermined range or not, based on the load weight and the lifting height, instead of the pressure.
According to the present invention, even though the load weight moment cannot be measured by the pressure acting on the tilt cylinder when the tilt cylinder is positioned at the stroke end, it can be determined whether a vehicle is stable or not because loading condition of the attachment can be found from the tilting angle of the tilt cylinder, the load weight and the load height by detecting the stroke end of the cylinder and the lifting height.